The electrocardiograph (ECG) signal describes the electrical activity of the cardiac muscle as it generates the various stages of the heart wave. Each cycle in the ECG signal may be subdivided into segments corresponding to stages of the heart wave, such as the P wave, the QRS complex, the T wave, the ST segment etc. Thus, the P wave of the ECG signal is due to depolarization of the atria, the QRS complex to depolarization of the ventricles, and the T wave to repolarization of the ventricles. Detection of an altered ECG signal is an important non-invasive tool in the diagnosis of cardiac abnormalities. Analysis of the ECG signal usually focuses on the ST segment due to its low-noise and its well-known correlation with cardiac abnormalities such as coronary artery disease (CAD). As physical stress is known to introduce features into the ECG signal indicative of CAD not present in signals obtained at rest it is common to obtain an ECG signal from a subject during a stress test comprising phases of rest, exercise and recovery from exercise.
In order to obtain sufficient information to provide usefull diagnostic information from ECG measurements it is thus current practice to obtain data
A high level of alignment is a prerequisite for any effective averaging process. Thus, filtering the signal, before performing the alignment, as suggested in the above method, makes it practically inapplicable to signals, such as the HF component of ECG, where the signal to noise ratio in the frequencies of interest is very high.
Thus the methods known in the art are not effective in the analysis of signals with a varying level of relatively high noise, especially when transient changes in the signals are of importance. A typical example of such a signal is the HF ECG component obtained during the above-mentioned exercise test, which has been shown to be of importance in early detection of ischemia—it has been shown by Beker et al. (Proceedings on computers in Cardiology, IEEE Computer Society, 33–35,1992) and Beker et al. (Pacing and Clinical electrophysiology 12:2040, 1996) that decrease in the energy level of the HF component of the QRS interval during the course of the exercise test may be indicative of ischemia.
There is therefore a need for a method to enhance the signal-to-noise ratio in periodic or semi-periodic signals, such as ECG signals—a method that will eliminate or substantially reduce the disadvantages of the prior art methods.